1. Field of the Invention
The present invention relates to water quality measurement for assessing an effect of water, which is fed to a water treatment membrane, on occlusion of the membrane.
2. Description of the Related Art
Various kinds of membranes are used to separate a component contained in water. There are two water filtration methods using the membrane. One of the methods is a dead-end filtration method or a method of passing an entire volume of feed water through a membrane. A component having failed to pass through the membrane is accumulated on the membranous surface. The other method is a cross flow filtration method, wherein water flows in parallel with the membranous surface and part of the water passes through the membrane. Among the membranes, a microfiltration membrane, an ultrafiltration membrane, a nanofiltration membrane, and a reverse osmosis membrane may be used for cross flow filtration. According to an ideal cross flow filtration method, a permeated water volume takes on a certain value determined with a water flow velocity but does not depend on an operation time. A component of raw water that does not permeate through a membrane is condensed and discharged. In contrast, according to the dead-end filtration method, a component of raw water that does not permeate through a membrane is accumulated. Therefore, a permeated water volume decreases along with the passage of an operation time. This phenomenon is called fouling. Even in the cross flow filtration method, since the component is adsorbed by the membrane, a phenomenon that the permeated water volume decreases is manifested. The phenomenon is also called fouling.
A related art will be described below by taking a reverse osmosis membrane, which is employed in advanced wastewater treatment, for instance. A semipermeable membrane is adopted as the surface of the reverse osmosis membrane, and the material thereof falls broadly into a cellulose triacetate series and an aromatic polyamide series. The membrane made of either of the materials allows only water to pass through by utilizing for filtration a difference in intensity of interaction between water molecules and dissolved components, and is used for the purpose of removing electrolytes in water.
The reverse osmosis membrane made of the aromatic polyamide series has high water permeability and exhibits excellent electrolyte removing performance, and is therefore widely used for industries. As for the structure of the reverse osmosis membrane, the structure of a complex membrane having a polyamide membrane, which has a thickness of 0.1 μm or less, formed on a microporous support whose thickness ranges from several tens of micrometers to several hundreds of micrometers is often adopted. The reverse osmosis membrane is used to remove dissolved organic substances or electrolytes during desalination of seawater, manufacture of pure water necessary to fabricate precision electronic devices such as semiconductors, advanced clean water treatment, or final treatment of wastewater or drainage.
For the final treatment of wastewater among the foregoing usages, water is generally fed to a reverse osmosis membrane through a treatment process described below. First, bulk trash and refuse contained in wastewater are removed through a screen. Thereafter, microscopic suspended substances including sand are settled in a sedimentation tank by applying, if necessary, a coagulant or the like and thus separated. Suspended solids and dissolved organic substances are contained in supernatant water, and are therefore biodegraded using microorganisms. Metabolites of the microorganisms are generated as sludge, and the sludge and water are separated from each other while being sedimented in the sedimentation tank or being passed through a microfiltration membrane. A primary effluent of the thus treated wastewater hardly contains suspended solids. In this state, disinfection or the like is performed in order to purify the effluent to such an extent that the water quality becomes high enough to sluice the effluent to a river. In Japan, in this state, treated wastewater is sluiced to a river, and water circulation is accomplished by utilizing natural purification. However, the Middle East, continental inland, or an island devoid of a river does not have a river or lake large enough to achieve the natural purification. Therefore, there is an increasing demand for reuse of the primary effluent of wastewater as drinkable water or industrial water by further purifying the effluent. The reverse osmosis membrane is used to remove dissolved organic substances or electrolytes from the primary effluent of wastewater during final treatment.
The primary effluent of wastewater contains organic substances that are measured as a total organic carbon (TOC) content ranging from 5 mg/L to 20 mg/L, though it varies depending on treatments performed in stages ending with a preceding stage. When the organic substances are separated from water using the reverse osmosis membrane, the organic substances can be reduced down to 1 mg/L or less. The TOC content is one of indices signifying water quality, indicates a total content of carbon in an organic compound among carbon compounds dissolved in water, and represents the total content of organic substances without identifying the components.
The reverse osmosis membrane employed in final wastewater treatment may be folded like a shape called a spiral or formed like a hollow fiber in order to increase a membranous surface area in a module. The spiral structure has a saclike reverse osmosis membrane fixed to the central core portion thereof and has it furled like an umbrella and stored in a cylinder. The mainstream of the module has a cylindrical shape having a diameter of 4 or 8 inches and a length of 1 m.
Adsorbates on a membranous surface include scales deposited due to a rise in the concentration of electrolytes in the vicinity of the membranous surface, a bio-fouling caused by microorganisms grown on the membranous surface, and an organic fouling caused by adsorption of organisms. The adsorbates are removed by regularly pouring cleansing water to the membranous surface or by utilizing a shearing stress. However, when organic substances are adsorbed by the membranous surface, they cannot be fully removed by the shearing stress but are gradually accumulated to make it necessary to replace the reverse osmosis membrane module with a new one. When the reverse osmosis membrane module is replaced with a new one, it is necessary to cease operation for a prolonged period of time. In addition, since the reverse osmosis membrane module cannot be recycled, the reverse osmosis membrane module has to be replaced with a new one. This leads to a cause of an increase in a running cost.
In the past, a fouling index (FI) value has been used as a water quality assessment method for predicting a fouling on a reverse osmosis membrane. The FI value may be called a silt density index (SDI) value. An FI value measurement method is stipulated in the JIS K 3802. Namely, water is passed through a filter, which has bores of 0.45 μm in diameter and has a diameter of 47 mm, at 206 kPa according to the dead-end filtration method, and a ratio of a time necessary to filter 500 ml in an initial stage to a time necessary thereto after the water is passed for fifteen min is calculated as the FI value. Manufacturers of the reverse osmosis membrane determine the upper limit of the FI values of feed water, and demand that the FI value should range from 3 to 4.